FIND-RULES-OF-RUNE

This function finds all the rules in wrld with :rune rune. It
returns a list of rules in their internal form (generally as
described by the corresponding defrec). Decyphering these rules is
difficult since one cannot always look at a rule object and decide
what kind of record it is without exploiting many system invariants
(e.g., that :rewrite runes only name rewrite-rules).

At the moment this function returns nil if the rune in question is a
:refinement rune, because there is no object representing
:refinement rules. (:refinement rules cause changes in the
'coarsenings properties.) In addition, if the rune is an
:equivalence rune, then congruence rules with that rune will be
returned -- because :equivalence lemmas generate some congruence
rules -- but the fact that a certain function is now known to be an
equivalence relation is not represented by any rule object and so no
such rule is returned. (The fact that the function is an
equivalence relation is encoded entirely in its presence as a
'coarsening of equal.)